the other important thing to remember about selecting the proper ethernet cable to connect your cable modem to router or your DSL modem to your router is to ensure that you have the right kind of category of cabling.
While most modems only have a 100 Mb per second network capability, connections between the router and other computers can be capable of more speed depending on the make and model and the manufacturer.
1 Gb per second routers cables and network interface adapters are available on the market to help speed up things like file transfers. In the case that you have equipment people of this kind of network speeds will need to ensure you have at least a Cat-5e network cable or a Cat-6 network cable.
If you intend to use a Cat-5 network cable you only be able to achieve 100 Mb per second speeds on your network.